It is a long-standing issue that medical implants made of metallic materials such as titanium (Ti) alloys and steels suffer unsatisfactory corrosion resistance and mechanical strength, and service life time much less than 10 years. Although pure titanium (wt. %>99%) or less alloyed titanium has relatively low mechanical strength (<350 MPa) and unsatisfactory fatigue behavior compared to those of titanium alloys, its in vivo and in vitro biocompatibility has been well proven to be unparallel by any titanium alloys and most other metals and alloys (such as steels) used for medical applications. Titanium with ultra-fine microstructure has been proved to have much improved mechanical strength and fatigue resistance than those of coarse-grained titanium. Besides, a lot of medical trials suggest that ultrafine-grained (UFG) Ti has improved biocompatibility. It is generally believed that nanostructured Ti with a mean grain size below 100 nm is more preferable for medical implant applications.
There are critical outstanding issues that have hindered the nanostructured Ti with outstanding mechanical properties and biocompatibility to be synthesized, and its subsequent development for structural and medical applications. The most important one is how to effectively prepare bulk titanium with dense nanostructures in a consistent and reliable manner. The currently known technique to prepare bulk nanostructured Ti is severe plastic deformation (SPD), which has two major limitations: (1) most of the grains of these nanostructured Ti have their sizes typically in the UFG regime (˜100-1000 nm), i.e., beyond the strongest size regime (tens of nanometers) of nanostructured metals; (2) the as-prepared UFG-Ti or nanostructured Ti has low ductility (elongation to failure <10%). Therefore, how to prepare bulk nanocrystalline Ti with grain sizes less than ˜100 nm and large ductility is a current challenge.
Current SPD technology cannot effectively prepare bulk pure titanium with a mean grain size smaller than 100 nm at room temperatures or elevated temperatures.
Although there are attempts in using bulk pure titanium with nanostructured surfaces manufactured by laser sintering, surface mechanical attrition treatment or plasma etching, the reliability and the resistances to corrosion and fatigue of these surface nanostructured titanium under the complex human body fluid conditions are questionable. Bulk nanostructured pure titanium not only could have enhanced mechanical strength and biocompatibility, but also could have better corrosion and fatigue resistances simply because they have much less impurities as compared with those of titanium alloys or steels.
There is a need in the art to have methods for fabricating a bulk nanostructured pure titanium with a mean grain size smaller than 100 nm.